There are numerous methods known in the art for producing a master printing plate, on which an image is written and which is then used as a printing plate for the reproduction of multiple copies Examples of such methods are described in "Chemistry and Technology of Printing and Imaging Systems", edited by P. Gregory and published by Blackie Academic & Professional in 1996. Typically, the plate contains one or more coating layers applied to a metal or plastic substrate layer.
The cost of producing a plate is relatively expensive and is generally only economical when utilized for printing large numbers of copies. For short printing runs, the cost of the printing plate adds substantially to the cost per printed copy, The plate cost is contributed from two sources:
a. The price of the plate itself PA1 b. The price of preparing the plate for printing, i.e. film making, exposure, processing. PA1 a) providing a cylinder having an affinity for ink; PA1 b) preparing a mixture including in ink adhesive polymer, a cross linking agent an infra-red absorbent agent and other appropriate ingredients to form an imaging layer; PA1 c) coating the prepared mixture on to the cylinder; and PA1 d) solidifying the prepared mixture. PA1 a) providing a printing member which includes a cylinder having an affinity for ink and an oleophobic imaging layer coated on the cylinder; or PA1 providing a printing member which includes a cylinder having an affinity for ink and a hydrophilic imaging layer coated on the cylinder; or PA1 providing a printing member which includes a cylinder having an affinity for water and an oleophillic imaging layer coated on the cylinder; and PA1 b) placing an image on the printing member. PA1 preparing a mixture including the appropriate polymer and a cross linking agent; PA1 coating the prepared mixture on to the cylinder; and PA1 solidifying the prepared mixture.
Recent developments in offset lithography have led to the use of digitally imaged printing plates whereby information is transferred directly from a computer to the printing plate. Though these printing plates are relatively easily prepared and quickly imaged and processed, their cost is even higher than that of conventional plates, so that they still contribute a significant cost to the printing price.
Another significant contributing cost factor in printing is due to what is commonly termed "make-ready". Make-ready refers to the operational stage involved between producing the last copy of one printing job and the first copy of the next job. Reducing the make-ready time improves the efficiency and allows for better utilization of machine time and increases the capacity of the machine.
Color printing generally involves the separating of the color information into four or more color components each on a separate printing plate and then superimposing the images printed from each plate on top of one another on each piece of substrate. In complex color printing, there are additional problems of lining up images on plates and ensuring that the color balance on the printed copies is correct, which can require more time and thus results in a further increase in the cost per copy.
Another time-consuming stage in conventional ("wet") offset printing is the fine adjustment of the balance of the fountain solution with the ink. This procedure not only is time consuming, but also requires a printer skilled in the art. In addition, the use of fountain solution also causes other problems, such as longer drying times and lower optical density. A waterless printing process for offset printing, which eliminates the use of fountain solution, is described in U.S. Pat. No. 3,511,178 to Curtin, A layer of silicone is used to repel the printing ink instead of the fountain solution.
Printing machines have been developed to minimize the make-ready by imaging directly on press. Infra red imaging has been used for this purpose because it lends itself to digital imaging and can be done under daylight conditions, For instance, the 74 Karat offset printer, manufactured by Karat LP, 3, Hamada Street, Herzelia, Israel, carries such a digitally imaged infra red system of plate production.
Besides the plate cost issue for short runs as mentioned above, the use of a printing plate has other disadvantages. It requires mechanical clamping devices at each end, which produces an unusable area on the plate cylinder as well as requiring the necessity of alignment mechanisms.
Various processes, known in the art, have been introduced for printing which do not require the use of a printing plate. For example, as described in "Chemistry and Technology of Printing and Imaging Systems", edited by P. Gregory and published by Blackie Academic & Professional in 1996, a printing process which may be termed "image one--print one" regenerates an image for each print. Ink jet printing whereby a jet of ink directly sprays the image onto the plate where the information is digitally applied from a computer is an example of an "image one--print one" process. This process is not competitive with high quality, color process printing using a printing plate such as offset lithography, because it is relatively slow and has severe substrate limitations.
Xerographic copying is another example of an "image one--print one" process. Disadvantages of this process, which may be considered as an imaging on press process, includes its complexity and the relatively high cost per copy that remains almost constant, irrespective of the number of copies made. Furthermore, this process has a generally inferior quality compared to lithography.
Numerous attempts have been made to produce a re-usable imaging surface for a printing process, examples of which are described in U.S. Pat. Nos. 5,206,102; 5,129,321; 5,188,033; 3,741,118; 4,718,340; 5,333,548 and 5,213,041. Generally, the above-mentioned systems generate a "master" which is then used for conventional wet offset printing.
Reference is now made to FIG. 1, which is a cross-sectional view of a printing member, referenced 300, used in existing conventional digital offset lithographic printing systems. The printing member 300 is formed of at least three layers, A first or substrate layer 310, forms a base or substrate for the printing member 300. A second radiation absorbing layer 312, that carries the image to be printed (once the printing member is imaged by ablation, for example), is over the first layer 310. A third surface coating layer 314 is over the second layer 312. Generally, the imaging layer 312 comprises an infra-red radiation absorbing material, for absorbing infra red radiation to cause ablation. The substrate 310 has an oleophillic surface. The surface coating layer 314 is of a material with an affinity for the ink(s) substantially different to the affinity for the ink(s) of the surface of the substrate 310. Ablation results in de-bonding between the surface coating layer 314 and the substrate 310. On cleaning--either dry or with a liquid--the materials of layers 312 and 314 are Removed in the image areas, revealing the surface of 310.
It would be advantageous to have an offset printing process which does not require a printing plate. Specifically, it would be of further advantage if such a process could be used in a waterless application. Imaging would be on the printing press and preferably, any processing after imaging would be relatively simple. U.S. Pat. Nos. 5,440,987; 5,634,403; and 5,636,572, all to Williams et. al, describe a seamless offset lithographic printing members. The printing members include a hollow cylinder which is attached to the cylinder jacket of an offset printing press. A polymeric coating layer is coated on to the cylinder and a second polymeric surface layer is coated on top of the first layer. While these patents address the problem of the void area needed for clamping plates on a cylinder, their inventions require a cylinder or cylinders to be removed from the printing press and then to receive two or more coatings before returning to press.
U.S. Pat. No. 5,713,287 to D. Gelbart, describes a plateless process at which a solvent-based, polymeric coating layer is deposited on-press on the cylinder. After drying, the imaging converts at least part of the coated layer to have an opposite chemical property to that of the layer.